The landscape of the city changes with each passing day as retail outlets come and go and older buildings are torn down to make way for new developments. But if you want to see what NYC looked like way back in the day, the Mannahatta Project has an interactive map that allows you to zoom in on any section of Manhattan—such as Union Square, left—and then travel back in time to see what the same area looked like in 1609.
The map isn't as exciting as you might expect. Manhattan was one big forest back then, so it doesn't matter if you focus on Union Square or Times Square—all you'll see is dense thicket of trees and brush. (Which is the point, of course; the Mannahatta Project is funded by the Wildlife Conservation Fund.)
But let's hope someone comes along and does the same sort of thing 50 years from now. That way your grandkids will get to see what Union Square was like during the rustic days of early '09—back when there was a store on the corner of 14th and Broadway that sold little pieces of plastic with music on them, and before a massive TGI Friday's took over the area. They'll never believe you otherwise!